1. Field of the Invention
The present invention refers to a supply device of circuit branches with LED diodes.
2. Description of the Related Art
Liquid crystal displays are widely used in mobile telephones; said displays use a large number of LED diodes to permit the phenomenon of backlighting. The LED diodes are distributed in the displays uniformly and use the same bias current; to obtain this they are connected in series.
To feed serially connected chains of LED diodes that emit white light, devices suitable for increasing the supply voltage above the value of the supply voltage at their-input are employed.
The most adopted circuit solutions provide for the use of a boost converter which, supplying many branches connected in parallel and each one made up of a series of LED diodes, permit the setting of the current or the voltage on each one.
To regulate the current that passes through one or more branches of LED diodes there are two different modes: a current one and a voltage one.
In the first mode only the current of the main branch can be set. The output current is read and compared with a reference to generate a control in pulse width modulation (PWM) mode; the circuit branches that are not controlled directly can even have a current very different from that of the main branch.
The disadvantage lies in the parallel connection of the circuit branches. Even if the current that flows in the main branch with the highest number of diodes is controlled directly, the secondary circuit branches can have an additional voltage and a different current. Adding a series of resistances in the secondary branches the current set on the main branch can be reached seeing that the resistances compensate the voltage jump error between the main branch and the secondaries that is due to the connection in parallel. In any case even if the object is reached a consistent quantity of power dissipation (on the compensation resistances) causes the decrease in the efficiency of the control.
This disadvantage can be present not only when supplying the circuit branches with a different number of diodes, but also if the number of LED diodes is equal in all the branches. In fact the voltage jump between the LED diodes could be different even if the same current flows. As a consequence it is necessary to impose a different voltage jump for each branch, but this is not possible by connecting all the branches in parallel. Only by regulating the current that flows through the circuit branches with a maximum value of voltage jump and inserting variable resistances in the other circuit branches the parallel connection can be maintained.
The voltage mode provides for the setting of the output voltage for each circuit branch by means of a boost converter and a voltage divider. To control the current that flows through each circuit branch a resistance, connected in series to the LED diodes, is added to each circuit branch; said resistance enable the current required to be adjusted. Nevertheless the value of the current cannot be known in advance given that it depends on the voltage at the terminals of the circuit branches, on the number of LED diodes present in each branch and on the fall in voltage on each LED diode; the latter depends on the flow of current and on the process technology. Therefore the correct resistance value must be assessed in the different cases and must be set so as to compensate the variation of voltage due to the process technology.